What English version should I get?

We must first realize that there is no 'perfect' translation from one
language into another. With Bible translation, not only are we translating from
one language into another, but we are also translating across significant
cultural boundaries. How does one express something written thousands of years
ago in a completely different culture in the thought and language of 21st
century English speakers? Second, there are so many English versions because of
a some important considerations that inform the translation of the original
Hebrew and Greek.

The Hebrew text of the Old Testament (OT) has been rather well
established. There are some uncertain texts in the standard Hebrew text, and
the evidence from the Dead Sea Scrolls, the Septuagint, the Samaritan
Pentateuch, and the Aramaic Targums shows that there were competing textual
traditions. Nonetheless, the Hebrew text that forms the basis for most English
versions is largely fixed. Some of the Hebrew words or phrases may be unclear,
and translators have to make some decisions about what the text meant. For
Christians, another factor that comes into play is that the New Testament (NT)
authors were dependent (in varying degrees) upon an ancient Greek translation
of the Hebrew Bible called the Septuagint. Some English translations of the OT
reflect this dependence.

Isaiah 7.14 is the text to check, and you can see a selection of English
versions HERE.
What you will note is that a number of versions say that a "virgin
shall conceive" (KJV and similarly, NIV, NASB, NLT...) while others say a
"young woman is with child" (NRSV and similarly NET and BBE)
while The Message gives a fuller rendering as "A girl who is presently a virgin will get pregnant."
The Hebrew word really only indicates a "young woman," and generally
this would imply a virgin, but it is not the specific term for that concept.
When the Hebrew was translated into Greek (and even later into Latin),
however, a word that does specify a "virgin" was used, and it is
this word that is used in Matthew 1:23 when the birth of Jesus is described.
The 'best' translation of the Hebrew, therefore, is probably "young
woman," but other factors have led some English versions to use
"virgin." A good version will at least provide a footnote to let the
reader know what is going on in the text.

The Greek text of the New Testament poses considerably more
difficulties. Keep in mind that the authors of the NT did not think of
themselves as writing Scripture in the same sense as they regarded the
Scriptures (i.e., the OT). There was, therefore, a bit more flexibility in the
transmission of the text in addition to the usual problems posed by the act of
hand copying a new text. There are literally thousands of variant readings in
the Greek NT, but most of them are quite insignificant. Still, there emerged a
number of 'families' of related Greek texts. One family is known as the
Majority Text (or Textus Receptus), and it is important because it was
used as the basis of the King James Version (KJV) in 1611 which has been a
reference point for every English version since then. Today, we have access to
many more ancient Greek manuscripts, and so, many scholars have tried to use
all the evidence to reconstruct a Greek text that they argue is closest to the
originals. Some English versions rely on the Majority Text or defer to its
readings (KJV, NKJV...) while others base their translations on the
reconstructed Greek text (e.g., NRSV, NET).

Mark
16.9 is one passage to check to see which Greek text is being used. The
best and oldest Greek manuscripts end with verse 8, but it has been a
puzzling ending to many readers, both ancient and modern. Some have
suggested that Mark the author somehow was kept from finishing his text or
that the last page of the book was lost. Some Greek manuscripts do indeed
have an ending, either a 'shorter' or 'longer' one. This longer ending is
part of the Majority Text tradition, and so the KJV goes from verse 8 to
verse 9 without any acknowledgement of the textual questions involved. The
English versions handle it in a variety of ways, but the 'best' version
should acknowledge that our best manuscripts end at verse 8 but that there
are other later endings that are known.

Another text that has similar issues is John 7.53-8.11, the account known
as the "Woman Caught in Adultery." It is a great story, and it
likely is a reliable story about Jesus... but the best Greek manuscripts do not
include it all, and others add it after John 7.36 or after John 21.25, and
in some manuscripts it appears after Luke 21.38. It was included here in the
Majority Text tradition (and so you will see it in the KJV), but most modern
English versions will want to let the reader know that it is not original to
the text.

Is a committee or an individual doing the translation? The strength of an
individual translation is that you tend to get a more consistent 'voice' and
perspective across the Scripture. (Given the variety of writings in the
Bible, that may or may not be a good thing.) The strength of a committee
project is that you tend to have the expertise of a wider range of scholars,
and questionable / difficult passages should reflect a consensus view.

Committees worked on the KJV, NRSV, NIV, REB, TEV, CEV…

An individual translated The Message and The Living Bible.

Do the translators reflect any particular theological perspective?
Ideally, one would not want to have any particular perspective slanting a
translation, but this really is impossible. The language one uses, the terms
that carry additional connotations, difficult passages that require choosing
some meaning... all these kinds of things mean that a perspective will be
reflected in a translation. As a reader you can choose a perspective that
reflects your own or perhaps one that will challenge it or choose something
like the NRSV which attempts to be ecumenical.

The NRSV intentionally was designed as an ecumenical translation which
included both Jewish and Christian scholars

Romans 1.16-17 provides a great example for discerning theological
tendencies. HERE is a PDF handout I
have composed that lays out the text interlinearly and then provides 14
translations. (The relationship between the translations is indicated by the
arrows.) What should you note?
In v. 17, how is the phrase "righteousness of God" rendered? Most
use that literal phrase, but note:

A righteousness from God (NIV)

God puts people right with himself (Good News)

God accepts everyone who has faith (CEV)

The saving justice of God (NJB)

God makes us ready for heaven—makes us right in God’s sight—when we put our
faith and trust in Christ to save us. (Living Bible)

How is the phrase literally rendered as "from faith to/for
faith" in the middle of v. 17 treated?
How is the quotation from Habakuk at the end of v.17 rendered? Especially
note the difference the word order makes. Is it "(The one who is
righteous by faith) will live"? Or "The one who is righteous (by
faith will live)"?

There are basically three approaches to translation, and the versions may
roughly be placed somewhere along a continuum of:

Formal Equivalence = “word for word”
Attempts to render every Hebrew/Greek word with an English equivalent - An
interlinear Bible is not really a translation but tries to show exactly the
Hebrew/Greek > English correspondence. The NASB and ESV would be
representatives of this approach.

Functional (or Dynamic) Equivalence = “thought for thought”Attempts to put into English language what the author would say if speaking to a modern audience -
The NLT, GNT, and CEV would be good examples of this approach.
(Note that this concern is often also reflected in the degree to
which inclusive language is used. I.e., when is it correct to use "man
/ men," and when would it be better to use "human / humans"?
How do you render the title traditionally known as the "Son of
Man"? How would the cultural context change matters? In an ancient
context where only men would be present, should the text indicate that? Or
should it reflect a modern setting where both men and women would be
present?)

ParaphraseAttempts to restate a passage in a personal or modern idiom (going even
further than a thought-for-thought approach) - The Message is the
best example of this approach.

Here is a chart that will give you a rough idea of how the translations
line up. (Read
the whole page for more descriptions. Do note that the versions which
appear exactly in the middle [NIV and TNIV] are published by the creators of
this chart!)

Note that the KJV stayed rather closely to the original Hebrew/Greek. The
NRSV, NAB, NJB, NIV, and TNIV are all good examples of versions that seek a
balance and try to practice the
explicit goal of the NRSV to be “As literal as possible, as free as necessary.”

Here are some examples of the formal / functional difference.

Matthew 20.9: "Each one received a denarius." (NASB) or "Each of them received the usual daily wage."
(NRSV) The Greek text uses the word "denarius," and the
translators must decide whether to use the literal term (and oftentimes
provide a footnote) or use a functional equivalent.

Matthew 14.24: "But the boat was already many stadia away from the
land... (NASB with footnote, "A stadion was about 600 feet") or
By this time the boat was a long way from the shore. (CEV)

Matthew 27.27 "Then the soldiers of the governor took Jesus into
the Praetorium" (NASB) or "Then the soldiers of the governor took Jesus into the governor's headquarters"
(NRSV with footnote, "Gk the praetorium)

Is the version intended for study purposes or to be read aloud in public
or for private, 'devotional' reading?
As you might discern, the more literal a translation is, the more awkward it
likely is for public reading. The more 'devotional' a translation is, the
less likely it is a literal translation.

What age group or reading level is intended?
Reading level is largely determined by sentence length and vocabulary.
According to a 1993 National Assessment of Adult Literacy,
the average adult reading level in the USA is 8th-9th grade. Here are some
approximate estimates of version reading levels:

What English-speaking audience is intended?
Even among English-speakers, there is sometimes a significant difference
between the English of the United States, Great Britain, and Australia. Most
of the versions cited above are oriented to the American market. The REB is
an excellent translation that reflects British English.

As an example of reading level and audience concerns, compare 2
Corinthians 1.12.

For our rejoicing is this, the testimony of our conscience, that in simplicity and godly sincerity, not with fleshly wisdom, but by the grace of God, we have had our conversation in the world, and more abundantly to you-ward.
(KJV)

Indeed, this is our boast, the testimony of our conscience: we have behaved in the world with frankness and godly sincerity, not by earthly wisdom but by the grace of God-- and all the more toward you.
(NRSV)

We can be proud of our clear conscience. We have always lived honestly and sincerely, especially when we were with you. And we were guided by God's wonderful kindness instead of by the wisdom of this world.
(CEV)

Now that the worst is over, we’re pleased we can report that we’ve come out of this with conscience and faith intact, and can face the world—and even more importantly, face you with our heads held high. But it wasn’t by any fancy footwork on our part. It was God who kept us focused on him, uncompromised.
(The Message)

As another example of reading level (and also functional translation
concerns), note that you will not find justification, righteousness, sanctification, redemption, atonement,
repentance, or covenant in the CEV. Why? In addition to being somewhat
technical, 'churchy' terms, they are terms not regularly used in everyday
language. Even the simple word "grace" is not used in the CEV (the
1995 edition; it did get added in the 2006 edition). The translators' research showed that the primary way most people used and knew
this term was as a reference to saying a prayer or to a graceful style.